The dopamine transporter (DAT) plays a critical role in physiological, pharmacological and pathological processes in brain. The transport system is a primary mechanism for terminating the effects of synaptic dopamine, thereby contributing to the maintenance of homeostasis in dopamine systems. It also appears to be a principal target of cocaine in the brain. (Kennedy and Hanbauer, J. Neurochem. 1983, 41, 172–178; Shoemaker et al., Naunyn-Schmeideberg's Arch. Pharmacol. 1985, 329, 227–235; Reith et al., Biochem Pharmacol. 1986, 35, 1123–1129; Ritz et al., Science 1987, 237, 1219–1223; Madras et al., J. Pharmacol. Exp. Ther. 1989a, 251, 131–141; Bergman et al., J. Pharmacol. Exp. Ther. 1989, 251, 150–155; Madras and Kaufman, Synapse 1994, 18, 261–275). Furthermore, the dopamine transporter may be a conduit for entry of neurotoxins into dopamine containing cells.
The striatum has the highest levels of dopamine terminals in the brain. A high density of DAT is localized on dopamine neurons in the striatum and appears to be a marker for a number of physiological and pathological states. For example, in Parkinson's disease, dopamine is severely reduced and the depletion of DAT in the striatum has been an indicator for Parkinson's disease (Schoemaker et al., Naunyn-Schmeideberg's Arch. Pharmacol. 1985, 329, 227–235; Kaufman and Madras, Synapse 1991, 9, 43–49). Consequently, early or presymptomatic diagnosis of Parkinson's disease can be achieved by the quantitative measurement of DAT depletion in the striatum. (Kaufman and Madras, Synapse 1991, 9, 43–49). Simple and noninvasive methods of monitoring the DAT are quite important. Depletion could be measured by a noninvasive means such as brain imaging using a scintillation camera system and a suitable imaging agent (Frost et al., Ann. Neurology 1993, 34, 423–431; Hantraye et al., Neuroreport 1992, 3, 265–268). Imaging of the dopamine transporter also would enable the monitoring of progression of the disease and of reversal of the disease such as with therapies consisting of implants of dopamine neurons or drugs that retard progression of the disease.
Other neuropsychiatric disorders, including Tourette's Syndrome and Lesch Nyhan Syndrome and possibly Rett's syndrome, are also marked by changes in DAT density. The DAT also is the target of the most widely used drug for attention deficit disorder, methylphenidate. The capacity to monitor the transporter in persons suffering from this disorder can have diagnostic and therapeutic implications. Furthermore, an age-related decline in dopamine neurons can be reflected by a decline in the dopamine transporter (Kaufman and Madras, Brain Res. 1993, 611, 322–328; van Dyck et al., J. Nucl. Med. 1995, 36, 1175–1181) and may provide a view on dopamine deficits that lie outside the realm of neuropsychiatric diseases.
The density of the DAT in the brains of substance abusers has also been shown to deviate from that in normal brain. For example, the density is elevated in post-mortem tissues of cocaine abusers (Little et al., Brain Res. 1993, 628, 17–25). On the other hand, the density of the DAT in chronic nonviolent alcohol abusers is decreased markedly. (Tiihonen et al., Nature Medicine 1995, 1, 654–657). Brain imaging of substance abusers can be useful for understanding the pathological processes of cocaine and alcohol abuse and monitoring restoration of normal brain function during treatment.
Accordingly, a radiopharmaceutical that binds to the DAT can provide important clinical information to assist in the diagnosis and treatment of these various disease states.
In order to be effective as an imaging agent for the disorders described above, it must have a specific binding affinity and selectivity for the transporter being targeted, e.g. DAT. Brain imaging agents must also have blood brain barrier (BBB) permeability. Yet, it has been difficult to produce a metal chelate which can cross the blood brain barrier while still retaining binding affinity and selectivity for its receptor site. Therefore, it is very desirable to find a suitable agent that satisfies these criteria and will complex with a desired radionuclide, such as 99mTc.
In addition, to be an effective imaging agent, a specific target:nontarget ratio is necessary. In the case of an agent selective for DAT one must take into account the fact that the striatum, the region of the brain having the highest density of the dopamine transporter, also contains serotonin transporter (SET). The SET is normally present at one-tenth to one-fifteenth the concentration of the dopamine transporter. Imaging agents that bind very strongly to DAT sometimes also exhibit a degree of binding to SET. Although such a nontarget binding typically poses no serious problem in the imaging of normal brains due to the greater number of DAT compared to SET, under disease conditions in which DAT are selectively reduced (or in which SET may be selectively increased), binding to the SET may make it difficult to quantify DAT. Moreover, binding to SET in other brain regions such as the hypothalamus and thalamus can reduce striatal contrast and diminish accuracy in localizing and imaging the striatum. Therefore, the target to nontarget binding ratio of DAT:SET can be important. Presently, among the most effective compounds for viewing and quantifying the DAT are phenyltropane derivatives that are labelled with positron emitters, such as 11C and 18F, and gamma emitters, such as 123I.
The radionuclide, technetium-99m, 99mTc (T1/26.9 h, 140 KeV gamma ray photon emission) is a preferred radionuclide for use in imaging because of its excellent physical decay properties and its chemistry. For example, its half-life of about 6 hours provides an excellent compromise between rate of decay and convenient time frame for an imaging study. Thus, it is much preferred to other radionuclides such as 123I, which has a substantially longer half life, or 18F, which has a substantially shorter half-life, and which are much more difficult to use. Its emission characteristics also make it easy to image. Further, it can be conveniently generated at the site of use. 99mTc is currently the radionuclide of choice in diagnostic centers around the world. It would be desirable to have a coordination complex with technetium for imaging DAT. Such a complex could be used for detecting conditions in which the DAT is useful as a marker.
However, a number of difficulties arise in the use of technetium for radioimaging agents because of its chemistry. For example, 99mTc must typically be bound by a chelating agent. Consequently it is much more difficult to design and prepare a 99mTc radioligand than it is to prepare a radioligand using other radionuclides such as 123I, which can be attached covalently to the ligand. The size of the chelating agent for technetium also can create problems when using this radionuclide in imaging agents. This can be an especially difficult problem when attempting to design receptor-based imaging agents using Tc.
Imaging agents being tested to determine their ability as diagnostic tools for neurodegenerative diseases typically are 123I labeled radioiodinated molecules. See, for example, RTI-55 (Boja, J. W., et al., Eur. J. Pharmacol. 1991, 194, 133–134; Kaufman and Madras, Synapse, 1992, 12, 99–111) or β-CIT (Neumeyer, J. L., et al., Med. Chem. 1991, 34, 3144–3146) and an iodoallyltropane, altropane (Elmaleh, D. R., et al., U.S. Pat. No. 5,493,026.
Although the tropane family of compounds are known to bind to the dopamine transporter, the addition of bulky chelating ligands for binding technetium or rhenium would be expected to affect potency and ability to cross the blood brain barrier of the resulting labeled complex. Kung, et al., in Technetium and Rhenium in Chemistry and Nuclear Medicine 4, eds. M. Nicolini, G. Bandoli, U. Mazzi, Servizi Grafici Editoriali, Padua, 1995, report that a 99Tc-labelled N2S2 ligand complexed with an arylpiperazine known to have selective binding to serotonin1A had only moderate binding affinity in vitro and failed to penetrate the intact blood-brain barrier.
It would be desirable to have a technetium or rhenium radio-labelled DAT imaging agent which is capable of crossing the blood brain barrier and has a high binding affinity and selectivity for the DAT.